One of the primary reasons that our arsenal of drugs has failed to lead to the successful control of tuberculosis is the ability of the etiologic agent, Mycobacterium tuberculosis (Mtb), to persist within the host tissues, where the drugs that readily kill this bacterium in vitro require prolonged periods of time to achieve similar effects. The extended duration of chemotherapy required to cure a human (6-12 months) is one of the greatest barriers to effective TB control. Poor patient compliance may lead to chronic infection, either active or latent, and the emergence of drug resistant Mtb. The underlying biology of persistent Mtb infection is poorly understood, and therefore, the objectives of this study are as follows: I) to better characterize the "persistent state" by examining drug efficacy and bacterial division at a variety of stages of infection in the mouse model; 2) to identify Mtb genes required for persistence in a mouse model using signature-tagged mutagenesis (STM); and 3) to identify Mtb genes that are required for the increased tolerance of Mtb to chemotherapeutic agents in vivo, again using STM in the mouse model. Achievement of these aims will provide a basis for the rational development of novel therapeutics that target persistent bacilli, thereby shortening the time required to achieve a cure.